I agree, this would be a really useful enhancement.
I ran into the same challenge while building expressions in flows and data action inputs. The only real way to validate them was to place them into the flow, make a test call, and then check the debug output. That works, but it can be quite time consuming when you are trying to troubleshoot formatting, variables, or conditional logic.
Having an expression tester during design time, where we could provide sample values and preview the output before publishing or testing a live call, would make development much easier and faster. It would also help reduce trial and error when building more complex expressions.
It may also be worth submitting this to the Ideas Portal, as I’m sure many in the community would run into the same challenge.